[gimp] plug-ins: minor fix of an error string.

[Jehan Pagès <jehanp src gnome org>]

commit 6936a655bdfb5c8ce4e9f683e2cbf04adf0a98e2
Author: Jehan <jehan girinstud io>
Date:   Wed Apr 4 02:21:47 2018 +0200

    plug-ins: minor fix of an error string.
    
    Was reading this part of the code when I saw this: the error happens
    when loading not exporting a PNG.

 plug-ins/common/file-png.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/plug-ins/common/file-png.c b/plug-ins/common/file-png.c
index 4619fdd..266dc09 100644
--- a/plug-ins/common/file-png.c
+++ b/plug-ins/common/file-png.c
@@ -876,13 +876,13 @@ load_image (const gchar  *filename,
   struct read_error_data error_data;
 
   pp = png_create_read_struct (PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
-  if (!pp)
+  if (! pp)
     {
       /* this could happen if the compile time and run-time libpng
          versions do not match. */
 
       g_set_error (error, 0, 0,
-                   _("Error creating PNG read struct while exporting '%s'."),
+                   _("Error creating PNG read struct while loading '%s'."),
                    gimp_filename_to_utf8 (filename));
       return -1;
     }